Maliks Muwatta Book 31, Hadith Number 82.

Yahya related to me from Malik from Abu’z-Zinad from Busr ibn Said from Ubayd, Abu Salih, the mawla of as-Saffah that he said, “I sold drapery to the people of Dar Nakhla on credit. Then I wanted to go to Kufa, so they proposed that I reduce the price for them and they would pay me immediately. I asked Zayd ibn Thabit about that, and he said, ‘I order you not to accept increase or to give to anybody.'”